Remove soon to be un-supported MacOS vagrant VMs.

@crwood mentioned on an internal channel that `macOS 10.15 and Ubuntu 18.04 both are nearing end-of-life (in ~2 months and ~8 months, respectively), and GitHub Actions is already dropping support for both of them (see 1, 2). It is cumbersome [for me] to run additional builders for these systems locally on the (very limited) hardware that I currently have.